This invention relates to retroreflective materials which are effective at high angles of incidence. More particularly, this invention relates to retroreflective material such as sheeting or film useful for marking surfaces which are positioned nearly parallel to light rays directed towards the surfaces.
Roadway markings, such as highway delineators, are commonly positioned parallel to the direction of roadway traffic. In such situations, reflection is maximum when incident light and the line of sight are perpendicular or nearly perpendicular to the surface of the reflective material. Although bead type retroreflective materials provide some degree of multi-directional retroreflectivity, the brilliance of the reflection does not adequately compare with that of cube corner type reflectors. The latter have directed retroreflectivity, that is, have the capability of high brilliance within a zone determined by the particular cube corner design. Whether of the cube corner or bead type retroreflective nature, retroreflective materials available to date exhibit progressively less reflectivity as the incidence angle of viewing light is increased. Generally, such materials lose nearly all of their reflectivity when the incidence angle becomes greater than about 60.degree.. Moreover, these materials are often dimensionally unstable, undergoing delamination under wet or changing weather conditions.
Representative of the present state of the art are U.S. Pat. Nos. 2,310,790 to Jungersen and 3,450,459 to Haggerty. Jungersen describes the use of mixed retroreflective elements or off-center cube corner elements (FIGS. 10-12, 15-16) to solve the problem of poor retroreflectivity at high incidence angles. Haggerty teaches the use of multi-faceted retroreflective elements to achieve reflection at high incidence angles. However, these approaches require a combination of different retroreflective elements and therefore are complex and tend to be difficult and expensive to achieve in practice.
The present invention provides a new and improved, dimensionally stable retroreflective material based upon retroreflective elements of a simple and single design. Such elements may be incorporated into films, sheeting or like materials, and are effective in any circumstance where it is desired to obtain reflection from a surface which is nearly parallel to incident light. As opposed to the "facing" retroreflectivity of conventional cube corner materials, the retroreflective materials of the invention provide highly efficient "grazing angle" retroreflectivity, achieving maximum retroreflectivity at angles of incidence of about 45.degree.-80.degree..